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My 10.1" works but my 7" doesn't?

Posted: Thu Jun 20, 2019 6:57 pm
by gigawat
Hello All,

I am new to the PI.

I have designed my own compute module carrier board with a TI SN75LVDS83ADGGR for LVDS. It works very well with my 10.1" 1280x800 LVDS display.

My config.txt is as follows:
---------------------
dtparam=audio=on

dtoverlay=dpi18-666-cff
hdmi_ignore_edid=0xa5000080
framebuffer_width=1280
framebuffer_height=800
enable_dpi_lcd=1
display_default_lcd=1
dpi_group=2
dpi_mode=87
dpi_output_format=0x6f217
hdmi_timings=1280 0 40 48 88 800 0 13 3 32 0 0 0 60 0 71000000 5
gpu_mem=256

dtoverlay=i2c-gpio,i2c_gpio_sda=28,i2c_gpio_scl=29,i2c_gpio_delay_us=2
enable_uart=1
---------------------
I have even used the following and it works perfectly:
---------------------
dtparam=audio=on

dtoverlay=dpi18-666-cff

hdmi_ignore_edid=0xa5000080
framebuffer_width=1280
framebuffer_height=800
enable_dpi_lcd=1
display_default_lcd=1
dpi_group=2
dpi_mode=87
dpi_output_format=0x6f217

gpu_mem=256

hdmi_cvt=1280 800 60 5 0 0 1

dtoverlay=i2c-gpio,i2c_gpio_sda=28,i2c_gpio_scl=29,i2c_gpio_delay_us=2
enable_uart=1
---------------------

My problem is when I switch to a 7" 1280x800 LVDS display. I get a black screen. No matter what I change, I get a black screen.

I have checked my hardware by doing the following:

I have a 10.1" HDMI to LVDS driver board that came with my 10.1" screen. It works great with the 10.1" screen. I am able to connect this same driver board with no changes to my 7" screen, with the same cable, and it works perfectly. I have confirmed the 7" screen is getting all of the various voltages it requires. Again, the same LVDS cable from my 10.1" HDMI to LVDS driver board works perfectly. So in my mind, disconnecting my LVDS cable going to my 10.1" display from my compute module carrier board and connecting it to my 7" display should work. I can tell the back light comes on, but there is just a black image. No raspberry pie symbols during the boot sequence. Nothing...

I have tried using the following:

hdmi_timings=1280 0 64 1 5 800 0 40 1 2 0 0 0 60 0 68200000 5

Which is based on the timings found in the 7" display data sheet, which I have attached here.
Would somebody please check and see that my hdmi_timings= parameters are matching what the data sheet is saying? Maybe that is my problem.

I don't know what else to check or try. Any ideas? This would be greatly appreciated. Thank you so much!